1. Field of the Invention
This invention relates to an improved chemical pressure vessel for storage, shipping and pressurized dispensing of fluid chemicals. The vessel is formed with a seamless fluoropolymer inner liner permanently encapsulated within a metallic overpack. To protect the liner from the heat of weld formation during the encapsulation of the overpack around the liner, the liner is formed with a recessed indentation immediately adjacent, that is, immediately behind or interior to the weld area, and the indentation retains a sacrificial layer of fluoropolymer to ensure that the heat of the welding will not affect the liner itself. The fact that the liner is seamless and is permanently encapsulated within the overpack eliminates the need for periodic disassembly of the vessel for inspection of seams for possible leakage.
2. Description of the Related Art
Various governmental agencies regulate the handling and transportation of hazardous chemicals in order to assure optimum protection of personnel and the environment against accidental spillage or leakage. Most pressurizable chemical vessels currently available for storage, shipping and pressurized dispensing of fluid chemicals are formed of a seamed inner liner encased within an outer metallic overpack, such that periodic disassembly of the overpack is required for inspection of the structural integrity of the liner against leakage. Certain pressurizable chemical vessels have been suggested with seamless liners, but none combine the features and advantages provided by the present novel seamless pressure vessel, as will be further described herein below.
Nisshin Gulf Coast EGC Corp. of Houston, Tex. provides a fluoropolymer coated metal vessel consisting of a fabricated (i.e., welded) inner fluoropolymer liner with a flanged stainless steel shell. The method by which NGC/EGC constructs their vessels is first by fabricating the outer metal (steel or stainless steel) jacket. This outer jacket would be equipped with either a bolt-on top or a manway large enough for a person to fit through. The lining is no more than pieces of sheet or film cut to the proper dimension, then rolled or formed to the proper contour of the inside jacket surface. These individual fluoropolymer pieces are then welded together inside the vessel. This lining may or may not be adhered to the outer steel/stainless steel jacket depending on the end-use (if used in vacuum application, adhering of the liner to the jacket would be necessary). The legend "NO WELDING PERMITTED" is used on the NGC/EGC vessel to prevent the possibility of someone welding on the tank that could result in the damage of the inner liner, from the high temperature of the welding arc.
The advantages of the present novel Seamless Pressure Vessel as compared to NGC/EGC are as follows:
a. Because the lining is fabricated from sheet stock and is welded together, there exists a greater potential for leaks at the welds. The inner liner of the Seamless Pressure Vessel of the present invention is a rotationally molded one-piece liner eliminating the potential for leaks at welds. The present novel vessel liner also allows a complete and thorough inspection of the liner before it is placed in the overpack. PA1 b. The only fusing/welding that is done on the present novel vessel is done on the outside of the overpack permitting easy inspection of these areas without having to disassemble the vessel. The NGC/EGC vessel would require the disassembly of that vessel and the inconvenience and safety hazard of requiring a person to crawl inside the vessel for inspection of the welds. PA1 c. The present novel vessel is currently United Nations approved for shipment of regulated materials internationally. NGC/EGC does not have such approval. PA1 d. The present vessel allows for complete drainage of chemical through the bottom port and 99.9% retrieval of contained chemical through the top discharge port. PA1 e. Incorporated in the present vessel overpack is a place where the overpack can be cut apart if the vessel is damaged and the more expensive inner liner could be salvaged. This is the same area where the final weld is made on the stainless steel overpack that incorporates the "puddle-plate" for protection of the inner liner, as will be further described herein below. PA1 f. The present vessel does not have threaded joints, as does the NGC/EGC vessel, which under pressure and over time could result in leaks. PA1 a. Delamination due to temperature cycling or a poor bond of the lining to the steel can cause gaps between the lining and the steel overpack. This type of gap has the potential to induce stresses that could ultimately lead to failure. PA1 b. The rotolining process does not lend itself to the use of PFA Teflon.RTM. due to the high shrink characteristics of PFA.RTM.. PA1 c. The rotolining process, as with the fabricating process of NGC/EGC, also makes inspection of the inner lining very difficult. The lining process again must take place after construction of the overpack.
Carlin, Jr. U.S. Pat. Nos. 4,625,892 and 4,699,294, describe a polyolefin lined tank in which the liner is formed by rotolining directly within the stainless steel overpack itself. Rotolining is a process by which the inner lining is sprayed on to the inside surface of the metallic vessel. This lining adheres to the steel as paint would if it were sprayed on. The liner is designed to shrink slightly away from the overpack after rotational molding, resulting in no adherence or bonding between the walls of the inner polyolefin tank and the outer metallic tank. This is intended to minimize damage to the tank through thermal expansion and contraction, the inner polyolefin tank and the outer metallic tank thus being free to expand and contract independently. In addition, it is said that the separation of the inner polyolefin tank from the outer metallic tank minimizes the potential for damage to the inner tank from any physical abuse to the outer metallic tank. The negative aspects to the rotolining as described by Carlin process are as follows: